The Lunar Orbital Platform-Gateway (LOP-G) is a planned lunar-orbit space station, that will have a power and propulsion system, a small habitat for the crew, a docking capability, an airlock, and logistics modules.

The station would be used as a staging point for lunar exploration and as a staging station for the proposed Deep Space Transport, which is a concept of a reusable vehicle that uses electric and chemical propulsion and would be specifically designed for crewed missions to destinations such as Mars.[1][6] If funded, the Gateway will be developed, serviced, and utilized in collaboration with commercial and international partners for use as a staging ground for robotic and crewed lunar surface missions and for travel to Mars.

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Originally, NASA had intended to build the Gateway as part of the now cancelled Asteroid Redirect Mission.[7][8] An informal joint statement on cooperation between NASA and Roscosmos was announced on 27 September 2017.[3] Traveling to and from cislunar space (lunar orbit) will help gain the knowledge and experience necessary to venture beyond the Moon and into deep space. The LOP-G would be initially placed in a near-rectilinear halo orbit (NRHO) around the Moon.[9] The Gateway could also support in-situ resource utilization (ISRU) development and testing from lunar and asteroid resources.[10] The Gateway would offer the opportunity for gradual buildup of capabilities for more complex missions over time.[11] If funded, its various components are to be launched on a commercial launch vehicle and on the Space Launch System as Orion co-manifested payloads on the flights EM-3 through EM-8.[9] According to Roscosmos, they may also use Proton-M and Angara-A5M heavy launchers to fly payloads or crew.[3]

The Power and Propulsion Element (PPE) for the LOP-G will have a mass of 8-9 t and be capable of generating 50 kW[8] of solar electric power for its ion thrusters system for maneuverability, which can be supported by chemical propulsion.[12] NASA stated that the most likely ion engine to be used on the PPE is the 14 kW Hall thruster called Advanced Electric Propulsion System (AEPS) still being developed by Glenn Research Center, the Jet Propulsion Laboratory, and Aerojet Rocketdyne.[13] Four identical AEPS engines would consume the 50 kW generated.[13]

Patrick Troutman serves as the lead for strategic assessments for the Deep Space Transport and the Lunar Orbital Platform-Gateway.[14]

On 1 November 2017, NASA commissioned 5 studies lasting four months into affordable ways to develop the Power and Propulsion Element (PPE), hopefully leveraging private companies' plans. These studies had a combined budget of $2.4 million. The companies performing the PPE studies are Boeing, Lockheed Martin, Orbital ATK, Sierra Nevada and Space Systems/Loral.[15][8] These awards are in addition of the ongoing set of NextSTEP-2 awards made in 2016 to study development and make ground prototypes of habitat modules that could be used on the Lunar Orbital Platform-Gateway as well as other commercial applications,[6] so the LOP-G is likely to incorporate components developed under NextSTEP as well.[8][16]

The Deep Space Gateway has received numerous criticisms from several space professionals for lacking a proper scientific goal. Former NASA Astronaut Terry Virts, who was a pilot of STS-130 aboard Space Shuttle Endeavour and Commander of the International Space Station on Expedition 43 wrote in an Op-ed on Ars Technica that the Deep Space Gateway would "shackle human exploration, not enable it". Terry stated that there is no concrete space human spaceflight goal with the Deep Space Gateway and that he cannot envision a new technology that would be developed or validated by building another modular space station. Terry further criticized NASA for abandoning its safety dictum of separating the crew from the cargo which was put in place following the Space Shuttle Columbia accident in 2003.[28]

Mars Society founder Robert Zubrin, who has been one of the staunchest advocates for a human mission to Mars, went even further and called the Deep Space Gateway "NASA's worst plan yet" in an article on the National Review. Robert went on to say "We do not need a lunar-orbiting station to go to the Moon. We do not need such a station to go to Mars. We do not need it to go to near-Earth asteroids. We do not need it to go anywhere. Nor can we accomplish anything in such a station that we cannot do in the Earth-orbiting International Space Station" and that "there is nothing at all in lunar orbit: nothing to use, nothing to explore, nothing to do". Robert also stated that "If the goal is to build a Moon base, it should be built on the surface of the Moon. That is where the science is, that is where the shielding material is, and that is where the resources to make propellant and other useful things are to be found."[29]

Retired aerospace engineer Gerald Black stated that the "LOP-G is useless for supporting human return to the lunar surface and a lunar base." He added that it is not even planned to be used as a rocket fuel depot and that stopping at LOP-G on the way to or from the Moon would serve no useful purpose and it would actually waste rocket fuel.[30]

Pei Zhaoyu, deputy director of the Lunar Exploration and Space Program Center of the China National Space Administration, concludes that, from a cost-benefit standpoint, the gateway would have "lost cost-effectiveness."[31] Pei said the Chinese plan to focus on a research station on the surface.[32]